Pertussis is a serious disease in infants which continues to occur. The currently available vaccine is effective but toxic. Improvement in the vaccine could be accomplished by inclusion of only those components of the organism instrumental in causing the human disease, but the identity of those components is not known. This project is directed at determining how B. pertussis attaches to the lining of the human respiratory tract and whether immunized children develop a means of blocking this attachment. Attachment is the first step required in order for the organism to cause disease. Delineation of the structure on B. pertussis by which attachment occurs is important both in understanding how the bacterium causes disease and in design of a less toxic vaccine. In view of the difficulty in obtaining ciliated cells from humans we began to study the mechanism by which B. pertussis attaches to ciliated cells in hamster tracheal organ cultures. The structures on B. pertussis most likely to be responsible for attachment to ciliated cells are fimbriae and/or glycocalyx. Both can only be visualized with the electron microscope. In a series of 20 experiments hamster tracheal rings inoculated with B. pertussis were stained and examined in the electron microscope. A new system was divised for infecting hamster rings which required that organisms attach to ciliated cells in order to infect. Electron microscopy of rings 48 hours after infection by this method revealed that structures resembling bacterial fimbriae were attaching the organisms to cilia and each other. This morphologic finding needs to be confirmed by determining whether removal or destruction of fimbriae prevents organisms from attaching to the cilia. In addition, whether fimbriae are the means by which B. pertussis attach to human ciliated cells and whether serum from immune children will block attachment will be examined in subsequent work under this project.